Surface Reactivity of Iron Oxide Pigmentary Powders toward Atmospheric Components: XPS, FESEM, and Gravimetry of CO and CO2Adsorption

Abstract

The adsorption of carbon monoxide and carbon dioxide (CO and CO2) on a number of specially prepared α-Fe2O3samples was measured gravimetrically at 25°C. The samples were prepared from a steel-pickling waste (97 wt% FeSO4·7H2O) by roasting the original material at 700°C for 5 h in air, oxygen, and nitrogen. Estimated surface coverages by the adsorbed CO and CO2were made on the basis of nitrogen-adsorption-based surface areas, while the nature of the sample surfaces was investigated by both X-ray photoelectron spectroscopy (XPS) and field emission SEM (FESEM) techniques. In addition a depth profiling study utilizing a sputtering argon beam and XPS was undertaken. Morphological studies using FESEM showed that neither CO nor CO2caused any significant structural changes. The nature of the resultant α-Fe2O3sample surfaces differed, with the degree of oxygenation decreasing in the order of preparatory gases: oxygen, (wet) air, nitrogen [IP(O), IP(A), and IP(N)]. The amounts of both CO and CO2adsorbed decreased in the sample order IP(A) > IP(O) > IP(N), though in the case of CO adsorption, the amounts adsorbed on IP(A) and IP(O) were not greatly different. In all cases the amounts adsorbed represented only fractional coverage. Adsorption of the more acidic CO2is thought to be favored more by basic Ox−2than by O2−sites on both IP(O) and IP(A), but with surface hydroxyl groups also playing a role (particularly on IP(A)). The CO2adsorption should result in the formation of mono-, di-, and polydentate carbonate and bicarbonate species, with increasing degassing temperatures favoring the polydentate species and the decomposition of the bicarbonate and carbonate to form undissociated CO2. The adsorption of CO (a weak base) is postulated to take place on strong Lewis acid, highly coordinated, metal sites to form metal carbonyl species, on strong base sites (O2−) to form carbonite, oxalate, and ketenic species, and, to a lesser degree, on surface hydroxyl groups to form formyl and formate species.